Today, solar energy is frequently considered as a renewable alternative to the energy generated by fossil fuel which is predominantly used today. Cost is a major factor in determining the type of energy source to be used, and it can reasonably be expected that when the energy created through the conversion of solar power is cost-competitive with that generated by fossil fuels, the use of solar energy will be more widespread.
Solar energy conversion modules converting sunlight into electrical energy typically use photovoltaic cells which directly convert solar energy into electrical energy. Photovoltaic solar cells are devices capable of transforming solar radiation into electricity in a direct manner. The amount of energy created by the cell is directly related with the amount of solar energy absorbed by the cell, and the amount of energy absorbed by the cell is a function of both the size and surface area of the cell and of the intensity of sunlight and wavelength striking it.
High-concentration photovoltaics (HCPV) is a new technology that is beginning to be a low-cost alternative for generating electricity.
The high cost of manufacturing photovoltaic modules, mainly the cost of cells, which are for the most part imported from other countries, mean that sale prices are excessively high.
In relative terms, the photovoltaic cell is the most expensive component of a solar energy converter. Therefore, the increase in the electrical production of the converter by increasing the surface area of the cells can be very expensive, and other methods are normally used to increase the intensity of sunlight striking the cell. Such methods include using concentrator lenses and/or mirrors for focusing the sunlight on the cell.
The size of the module also affects the cost in other less direct manners. Since most solar energy converters are manufactured far from their installation site, transport and final assembly costs can be considerable. Transport costs can obviously be greatly reduced by reducing the size of the module converter, and simplification of the general structure can be expected to reasonably reduce assembly costs and the cost of the solar collector itself.
In fact, to install a megawatt-peak of conventional photovoltaic modules in a semiconductor material, a space equivalent to the surface of a football field, i.e., approximately 8000 m2, is required. In contrast, in the case of high-concentration photovoltaics, the necessary semiconductor surface is reduced to eight square meters, which demonstrates the economic advantages of this technology, because the use of space for installations of high-concentration solar module farms or panels is much less.
With respect to the foregoing, it should be pointed out that conventional photovoltaic cells are manufactured with silicon, in contrast those used in high concentration are made with elements from groups III-V of the periodic table system, elements such as gallium, indium, phosphorus and others of the same type, usually on germanium substrates, forming multijunction tandem cells which allow using the solar spectrum much more efficiently.
In the case of silicon cells, since they are single-junction cells, the theoretical conversion limit, determined by their efficiency, is about 40% in concentration conditions. In contrast, for multijunction cells, the theoretical limit is 86.4%, so the improvement potential is very high.
Commercial silicon cells today have maximum efficiencies of twenty-one percent (21%) (monocrystalline silicon) for one sun, whereas triple-junction cells have efficiencies of about 37%.
Actually, most conventional silicon photovoltaic installations have efficiencies of less than 15%. Accordingly, the total photovoltaic solar capture surface can be drastically reduced by means of using high concentration photovoltaics (almost half that today, i.e., 50% of the surface required by conventional photovoltaics, and with the potential to even reduce this percentage). This reduction of total surface area required for installed equivalent peak power by means of using high-concentration photovoltaics technology allows reducing the cost of important elements of the installations, such as a lower amount of necessary terrain, a lower number of solar trackers, reduction of wiring distances and other structural elements, and reduction of transport costs as a result of the decrease in volume and weight of the required elements.
As a result of the foregoing, the cost per installed Watt has a huge reduction impact.
In some countries, such as Spain, greater priority is given to indoor electrical energy-generating photovoltaic installations than in solar plants, so technological advances must be aimed at said location.
A related object is to provide a solar energy converter of this type using an individual optical lens or concentrator complemented with a secondary optical element for each cell.
The system of applying solar radiation concentrator lenses on photovoltaic cells to increase the electrical energy production capacity thereof consists of using a concentrator lens made from glass, methacrylate, polyurethane, polyethylene, polypropylene or any other type of material of a similar class, which is transparent to allow the passage of solar rays. In this case, Fresnel lenses have the property of being high-powered solar radiation concentrator elements, and accordingly allow harnessing said energy in the field of energy photovoltaics.
Circular concentric grooves are engraved on said lens along the entire diameter of the lens, this being the element giving the lens its solar radiation concentration power. In summary, it is a conventional solar radiation concentrator lens the dimensions of which are usually between 10 to 30 centimeters in diameter, said measurements being able to vary according to the needs for which it is to be used.
The lens is located on a support or frame having a smaller double bottom to place the photovoltaic cell therein, located between 10 to 30 centimeters spaced from the concentrator lens. Once the assembly is oriented towards the position of the sun, the rays strike the lens, passing through it, until reaching the photovoltaic cell which receives said solar radiation increased in power due to the effect of a larger radiation surface upon its passage through the concentrator lens and the additional secondary optical element.
The units thus arranged, i.e., the assembly of a concentrator lens, superimposed on a photovoltaic cell at a distance of between 10 to 30 centimeters, and both elements supported on a box, casing or frame, can be placed in series to form the photovoltaic modules, and in the necessary number to reach the wattage that is to be determined in each module, taking into account the energy production capacity of each cell according to the higher performance obtained by the concentrator lens efficacy.
In addition, it is important to stress that unlike other technologies already tested in installations for many years, high concentration photovoltaics are not yet used in plants operating for a prolonged time. It is therefore fundamental to make devices which offer long-term reliability assurances.
Most high-concentration photovoltaic modules known on the market are closed-type modules, as shown in patent ES2229950, where a surrounding structure or casing having lenses on its upper outer surface contains active elements, such as cells, protection diode, and the necessary wiring.
The aforementioned elements of the modules are very sensitive to humidity and contact with it causes accelerated degradation which can limit its useful life under acceptable operating conditions. Even though systems are incorporated for encapsulating these elements, it is very important for the receptor to be sealed and to prevent the entrance of humidity or other external elements to prevent these effects.
Modules existing on the market have not fulfilled the necessary leak-tightness in a satisfactory manner, as is the case of patent ES2267382, the structure of which in addition to not assuring leak-tightness because it is formed by a central U-shaped sector and to side fins fixed by fixing means such as resins, in the event that a part therein should break or malfunction, it is necessary to break the module to access it. Likewise, a factor to take into account is the problem of relative humidity generated inside the module, having direct consequences on the active elements of the system.
In addition, the closures of current modules require using adhesive materials which prevent or complicate replacing lenses or other elements of the module.
Furthermore, high structural rigidity allowing the structure to perform suitably against the stresses it will have to withstand in the service life of the installation, which is about 25 years, is required for the system. The system will be exposed to the elements, withstanding extreme climate conditions. An international standard, IEC 62108, has been defined to simulate the behavior of the system, and compliance with this standard is mandatory for any high-concentration photovoltaic product that will be part of this market. This standard requires performing a series of tests that allow simulating the expected field behavior of the system.
A system providing high-concentration photovoltaics, preventing the drawbacks existing in earlier systems of the state of the art, is therefore desirable.